1. Field of the Invention
The present invention relates to an image receptor sheet for use in heat-melting thermal transfer. More specifically, it relates to an image receptor sheet for use in thermal transfer, which has the property of receiving thermal transfer images, patterns, characters, etc. (to be sometimes simply referred to as image(s) hereinafter), and which gives an image transfer product excellent in scratch resistance, abrasion resistance and weatherability.
2. Prior Art of the Invention
A heat-melting thermal transfer device is recently widely used in a facsimile machine, a word processor, a computer terminal printer, and the like since it has features in that it is noise-free because of its non-impact system, maintenance-free, less expensive, small in size and light in weight. A thermal transfer material is generally produced by forming a heat-melting ink layer composed mainly of a wax on one surface of a substrate formed of a thin plastic film such as a polyester film (e.g., polyethylene terephthalate or polyethylene naphthalate). An image is transferred as follows. While the above heat-melting ink layer is in contact with the surface of an image receptor such as general paper, part of the heat-melting ink is transferred to the image receptor by heating the other surface of the substrate, for example, by means of a thermal head.
With a recent progress in the automatization of factories and stores, thermal transfer materials are increasingly used in the fields of labels and barcodes, and image-recorded sheets (e.g., labels and barcodes) are increasingly required to have various resistances such as scratch resistance and abrasion resistance. For this purpose, there has been developed a thermal transfer material having a heat-melting ink layer (transfer layer) composed mainly of a resin, and various materials such as coated paper, synthetic paper and a plastic sheet have begun to be used as image receptors depending upon purposes. When the thermal transfer material having a heat-melting ink layer composed mainly of a resin is used, it is difficult to transfer the heat-melting ink to general paper. The above heat-melting ink layer can be transferred to synthetic paper or a plastic sheet, while the adhesion of the heat-melting ink layer to the synthetic paper or the plastic sheet is insufficient, and the heat-melting ink layer is easily peeled off when a Cellophane tape is attached and peeled.
As an image receptor which serves to improve the resistances of an image transfer product, JP-A-63-137892 discloses an image receptor sheet produced by forming a layer of a thermoplastic resin having a melting point of 135.degree. C. or lower on a substrate. However, the thermoplastic resins disclosed in the Examples of JP-A-63-137892 are polyolefins having a melting point of 100.degree. to 135.degree. C. such as polyethylene, and the layers of these thermoplastic resins are hence poor in adhesion to a substrate other than paper, such as a polyethylene terephthalate film generally used as an OHP film. Further, these thermoplastic resins have relatively high melting points, and are therefore poor in adhesion to a transfer image when the transfer layer is formed of a resin-containing heat-melting ink layer.
JP-A-1-120389 discloses an image receptor sheet produced by forming an image receptor layer composed mainly of a lubricant and a thermoplastic resin having Tg of 50.degree. to 100.degree. C. on a substrate. However, when the lubricant is natural wax, synthetic wax or higher fatty acid metal salt, the image receptor layer is poor in surface gloss. Further, since the lubricant is poor in weatherability, the image receptor layer is liable to undergo oxidation or hydrolysis to deteriorate when the image transfer product is used outdoors, and the image transfer product sheet practically discolors or deteriorates in gloss.
For improving the image receptor sheet in the image receiving performance and adhesion to a transfer image, generally, a thermoplastic resin having a melting point or softening point of 100.degree. C. or lower is used for forming the image receptor layer. Of such thermoplastic resins, generally, those having a sharp melting point are polymers having a low molecular weight (about 2,000 or less) or oligomers, and films formed of them show low film strength and have almost no weatherability. There is therefore a problem in practical use. That is, when an image transfer product including such an image receptor layer is used outdoors, the image receptor layer deteriorates in a short period of time. Thermoplastic resins which have no sharp melting point but show a softening point have tack (adhesion property) at a temperature between ordinary temperature (about 30.degree. C.) and a temperature around their softening points. Therefore, when the image transfer product is used outdoors, the image receptor layer is softened to show tack due to an increase in temperature caused by sunlight. As a result, dust may adhere or soot may be adsorbed to make the image transfer product dark and dirty. Further, when the image receptor layer is formed of a thermoplastic resin having a softening point of 100.degree. C. or higher, the image receptor sheet shows sufficient image-receiving performance or sufficient adhesion to a transfer image only when such high energy as will give an overload to a thermal head is charged to a thermal transfer material having a thermal transfer layer composed mainly of a resin. It has been therefore difficult to obtain a thermal transfer receptor sheet which exhibits sufficient image-receiving performance and adhesion, to a transferred image when used with a thermal transfer material having a transfer layer composed mainly of a heat-melting ink and which gives an image transfer product excellent in scratch resistance, abrasion resistance and outdoor weatherability.